1. Field of the Invention
This invention relates to an angle compensation method for a photodiode light-receiving surface in an inclination detection device wherein reflected light reflected off an object surface is received on the photodiode light-receiving surface divided into four parts and the inclination of the object surface is sought from changes in the position of irradiation of the reflected light on the photodiode light-receiving surface.
2. Description of the Prior Art
In the prior art, the inclination of the object surface was detected by an optical lever method in a pin-contact-type surface roughness tester, scanning probe microscope (atomic force microscope) or a two-dimensional position sensing detector with laser beam reflection.
FIG. 2 is a sketch for explaining the principle of detection of inclination of an object surface by the known optical lever method. In FIG. 2, the photodiode light-receiving surface D is disposed opposite the object surface P, which is the X-Y plane, and the reflected light L1 when the light beam L is incident on the object surface P is received by the photodiode light-receiving surface D. This photodiode light-receiving surface D comprises light-receiving surfaces D1, D2, D3 and D4 divided into four parts by an a-axis and a b-axis perpendicular to each other, and when the photodiode light-receiving surface D is projected on the object surface P, the position of the photodiode light-receiving surface D is adjusted such that the projected a-axis (projected a-axis below) corresponds with the Y-axis and the projected b-axis (projected b-axis below) corresponds with the X-axis, in which state the inclination of the object surface P is measured.
When the object surface P is rotated about the Y-axis by angle xcex3, the position of irradiation of the reflected light L1 on the photodiode light-receiving surface D is displaced along the b-axis depending on the amount of change, and when the object surface P is rotated about the X-axis by angle xcex4, the position of irradiation of the reflected light L1 on the photodiode light-receiving surface D is displaced along the a-axis depending on the amount of change. The light beam L in this case has an appropriate amount of spread, i.e., an appropriate amount of spread that can all be contained on the photodiode light-receiving surface D, and the spread of the beam is adjusted in advanced so that it will irradiate the center of the photodiode light-receiving surface D.
Here, when the object surface P is rotated up by angle xcex3 around the Y-axis by the actuator 16 and the center position of the irradiation of the reflected light L1 moves from the irradiation position M1 on one side toward the irradiation position M2 on the other side, this movement causes the light to become thinner on one side of the a-axis and to become thicker on the other side, and this trend becomes marked as the angle xcex3 becomes larger. That is, the difference A in the amount of light obtained by subtracting the amount of light received on one side of the a-axis (amount of light received on light-receiving surface D2 and light-receiving surface D3) from the amount light received on the other side of the a-axis (amount of light received on light-receiving surface D1 and light-receiving surface D4) is proportional to the angle xcex3 when the angle xcex3 is small, and therefore by detecting the light amount difference A, it is possible to seek the angle xcex3.
Similarly, when the angle xcex4 is small, the angle xcex4 can be sought by the difference B obtained by subtracting the amount of light received on one side of the b-axis (amount of light received on light-receiving surface D3 and light-receiving surface D4) from the amount of light received on the other side of the b-axis (amount of light received on light-receiving surface D1 and light-receiving surface D2).
However, in the above inclination detection method, agreement of the Y-axis with the projected a-axis and agreement of the X-axis with the projected b-axis are prerequisites for measurement, and if they do not agree, the data accuracy of the light amount differences A and B will be degraded, which will degrade the accuracy of measurement of the inclination of the object surface P. Next, degradation of the data accuracy of the light amount differences A and B is discussed using FIG. 3 through FIG. 6.
Degradation of the data accuracy of the light amount differences A and B can occur in the following two modes:
(I) When the photodiode light-receiving surface D is rotated about the c-axis, which passes through the intersection of the a-axis and the b-axis and is perpendicular to the a- and b-axes.
(II) When the photodiode light-receiving surface D is rotated about the Z1-axis, assuming this Z1-axis is parallel to the Z-axis of the object surface and is positioned behind the photodiode light-receiving surface D.
In the case of the mode in (I) above, as shown in FIG. 3, the a-axis projected on the X-Y plane (projected a-axis) is rotated about the Z-axis by angle xcex1 with respect to the Y-axis. If the object plane P is inclined by angle xcex4 in this mode, the irradiation of the reflected light L1 on the photodiode light-receiving surface D generates a locus on the Y-axis (axis inclined by angle xcex1 from the a-axis) projected on the photodiode light-receiving surface D as shown in FIG. 4. Also, when the object surface P is inclined by angle xcex3, the irradiation of the reflected light L1 on the photodiode light-receiving surface D generates a locus on the X-axis (axis inclined by angle xcex1 from the b-axis) projected on the photodiode light-receiving surface D as shown in FIG. 4.
Therefore, the light amount difference A on both sides of the a-axis is primarily proportional only to angle xcex3 and is therefore described by A=mxc2x7xcex3 (where, m is the compensation factor), but due to the occurrence of angle xcex1 (rotational shift around the c-axis of the photodiode light-receiving surface D), it is now described by equation (1).
A=mxc2x7(xcex3xc2x7cos xcex1+xcex4xc2x7sin xcex1)xe2x80x83xe2x80x83Equation (1) 
Further, the light amount difference B on both sides of the a-axis is primarily proportional only to angle xcex4 and is therefore described by B=nxc2x7xcex4 (where, n is the compensation factor), but due to the occurrence of angle xcex1 (rotational shift around the c-axis of the photodiode light-receiving surface D), it is now described by equation (2).
B=nxc2x7(xcex3xc2x7cos xcex1+xcex4xc2x7sin xcex1)xe2x80x83xe2x80x83Equation (2) 
In this way, the factor sin xcex1 interferes with the light amount differences A and B, and due to this factor, the data accuracy of the light amount differences A and B is degraded.
In the case of the mode in (II) above, as shown in FIG. 5, the line of intersection R between the X-Y plane and the photodiode light-receiving surface D is rotated about the Z-axis by angle xcex2 with respect to the Z-axis. If the object plane P is inclined by angle xcex4 in this mode, the irradiation of the reflected light L1 on the photodiode light-receiving surface D generates a locus on the Y-axis (axis inclined by angle xcex2 from the a-axis) projected on the photodiode light-receiving surface D as shown in FIG. 6. Also, when the object surface P is inclined by angle xcex3, the irradiation of the reflected light L1 on the photodiode light-receiving surface D generates a locus on the X-axis (axis inclined by angle xcex2 from the b-axis) on the photodiode light-receiving surface D as shown in FIG. 6.
Therefore, the light amount difference A is primarily proportional only to angle xcex3 and is therefore described by A=mxc2x7xcex3 (where, m is the compensation factor), but due to the occurrence of angle xcex2 (rotational shift around the Z1-axis of the photodiode light-receiving surface D), it is now described by equation (3).
A=mxc2x7(xcex3xc2x7cos xcex2+xcex4xc2x7sin xcex2)xe2x80x83xe2x80x83Equation (3) 
Further, the light amount difference B is primarily proportional only to angle xcex4 and is therefore described by B=nxc2x7xcex4 (where, n is the compensation factor), but due to the occurrence of angle xcex2 (rotational shift around the Z1-axis of the photodiode light-receiving surface D), it is now described by equation (4).
B=nxc2x7(xcex4xc2x7cos xcex2+xcex3xc2x7sin xcex2)xe2x80x83xe2x80x83Equation (4) 
In this way, the factor sin xcex2 interferes with the light amount differences A and B, and due to this factor, the data accuracy of the light amount differences A and B is degraded.
As explained above, it is possible to raise the data accuracy of the light amount differences A and B by deleting the interfering terms sin xcex1 and sin xcex2, which is equivalent to adjusting the posture of photodiode light-receiving surface D to make both angle xcex1 and angle xcex2 zero.
However, in adjusting the position of the photodiode light-receiving surface D as explained above, the adjustment that makes angle xcex2 zero (adjustment about the Z1-axis) has not been performed at all, and this becomes a factor that can degrade the measurement accuracy of the inclination of the object surface P.
Further, the adjustment that makes the angle xcex1 zero (adjustment about the c-axis) has been performed, but this adjustment is based mainly on the experience of the person performing the measurement, and therefore it cannot be said to be a highly accurate adjustment and may be factor in further degrading the measurement accuracy of the inclination of the object surface P.
This invention was proposed to address these issues, and its purpose is to offer an angle compensation method capable of improving the measurement accuracy of the inclination of an object surface in an inclination detection device.
In order to achieve the above purpose, this invention provides an angle compensation method for compensating for an angle of a light-receiving surface of a photodiode disposed in an inclination detection device, the light-receiving surface being divided into four parts by an a-axis and a b-axis disposed perpendicular to each other and receiving light reflected from an object surface that is an X-Y plane, the inclination detection device seeking an inclination of the object surface from changes in an irradiation position of the light reflected on the photodiode light-receiving surface, the method comprising the steps of fixing the light-receiving surface to a rotary stage that can rotate both about a c-axis that passes through an intersection of the a-axis and b-axis and is perpendicular to the a- and b-axes and about a k-axis that is parallel to a Z axis of the object surface; and rotating the light-receiving surface about the c-axis and k-axis so that, when the light-receiving surface is projected onto the object surface, the a-axis aligns with a Y-axis and the b-axis aligns with an X-axis.
The light-receiving surface can be fixed to a rotary mechanism about the k-axis supported on a rotary mechanism about the c-axis or to the rotary mechanism about the c-axis supported on the rotary mechanism about the k-axis.
Further, the rotary stage comprises a spherical magnetic substance and is housed in a sphere holder having three perpendicular walls, and the sphere is pulled by magnets disposed in the corners formed by the three walls to facilitate rotation of the sphere.
As described above, in this invention the light-receiving surface is fixed to the rotary stage, and the rotary stage is rotated about the c-axis and about the k-axis to eliminate any rotational shift of the light-receiving surface with respect to the object surface. Therefore, the projected a-axis aligns with the Y-axis and the projected b-axis aligns with the X-axis. This can facilitate prevention of degradation of the data accuracy of the light amount differences A and B, and greatly improve the measurement accuracy of the inclination of the object surface.
Further, since the rotary stage is preferably a magnetic sphere as described above and the sphere is pulled toward the corners by magnets while being held in the sphere holder, the sphere can be held stable and angle compensation of the photodiode light-receiving surface can be performed precisely.